School of Meteorology (Defense)

Climatology of RHICE Distributions in Cirrus Clouds by Synoptic Regime

Andrew Dzambo
OU School of Meteorology

24 April 2015, 2:00 PM

National Weather Center, Room 5820
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Cirrus clouds have long been known to impact the earth’s radiation budget. Characterizing cirrus clouds in climate models has also proven challenging over the years. Climatologies of cirrus clouds have been created in the past based on their inherent microphysics, relative occurrence through the year, and dynamics driving their formation. Our understanding of cirrus clouds is very good overall, however, little is known about how RH, specifically RH with respect to ice (RHICE), varies in cirrus clouds under different synoptic regimes. This study attempts to bridge this gap in our understanding by incorporating a sophisticated synoptic climatology with collocated radiosonde and millimeter wavelength cloud radar (MMCR) data at the Atmospheric Radiation (ARM) program’s Southern Great Plains (SGP) site.

Vaisala RS92 radiosondes have a well documented history of dry biased RH measurements (induced by solar radiation); therefore, to ensure the best possible RHICE measurements, two radiosonde RH dry bias correction algorithms – Wang et al. (2013 JTECH; WANG hereafter) and Miloshevich et al. (2009 JGR; MILO hereafter) – are tested against one another. An analysis with ARM microwave radiometer (MWR) precipitable water vapor (PWV) data reveals that both corrections are much less biased compared to the original RH data; however, both corrections are similar in terms of overall corrected PWV. Knowing that MILO corrects RH more in the upper troposphere compared to WANG, a pair of radiance closure experiments – one downwelling and one upwelling – is performed. The radiance closure experiments, after accounting for seasonal variations in upper-tropospheric water vapor and the inherent use of each algorithm, suggest that the WANG RH correction algorithm is the optimal choice for evaluating RHICE in cirrus clouds.

This study is looking at how mid-latitude RHice varies with different synoptic classifications. The synoptic climatology of the ARM SGP site was derived by Marchand et al. (Journal of Climate; 2009) and includes 21 classifications under 5 parent classifications. Key cirrus cloud RHICE results include variations of ice saturation according to parent classification and a very high frequency of cirrus cloud occurrence in regimes dominated by southerly flow. RHICE distributions in cirrus cloud tops for summer classifications are found to be statistically different from three of the other parent classifications, suggesting synoptic-scale weather systems as opposed to mesoscale convective systems are more responsible for higher ice saturation in cirrus clouds. Implications regarding ice nucleation mechanism (heterogeneous vs. homogeneous) and the ambient dynamics driving cirrus cloud formation (e.g. gravity waves, turbulence) are also discussed.

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